Preliminary Data shows increased IL-23 protein levels in human colon biopsies positive for C. difficile toxins compared to negative controls (p=0.008). Using two distinct murine models, mice lacking IL-23 signaling had a significant increase in survival (100% n=12) when compared to control mice (16.7-50%, n=12). Additionally, infected mice lacking IL-23 signaling did not induce the downstream cytokines IL-17 and IL-22 as infected wildtype mice do, indicating either or both cytokines may be involved in contributing to IL-23 dependent pathology during C. difficile infection (CDI). Approach: We propose to examine the mechanisms by which the IL-23 signal mediates pathogenesis during CDI using a murine model. Infected IL-23 mAb treated WT mice will be compared to IgG isotype WT mice. We first plan to elucidate whether IL-23 drives pathology by failing to clear bacteria, or through the ability to enhance inflammation and epithelial tissue destruction. This will be done by counting colony-forming units (CFU) of C. difficile in stool and scoring colon tissue histology, as well as measuring LPS and FITC-Dextran levels in serum. We will also analyze differences in immune cell recruitment, with an emphasis on neutrophils, through flow cytometry, in order to access the contribution of the host response to disease severity. We will establish effector cells responsible for receiving the IL-23 signal via the use of infected and uninfected IL-23R- GFP labeled heterozygote mice. Lastly, we will use flow cytometry to establish pathogenic downstream cytokines being produced in an IL-23 dependent manner during disease. We will neutralize each cytokine (starting with the most likely contributors to pathology IL-22 and IL-17) to determine their contribution to pathology. In order to correlate this work to human disease, we will individually stain C. difficile positive and negative human biopsies for each downstream cytokine shown to be important and evaluate protein expression. Significance: C. difficile is currently the leading cause of hospital-acquired infections in the US and, despite therapy, is the cause of approximately 14,000 deaths annually. This stresses the need to better understand mechanisms of disease severity and develop alternate therapies to treat disease. This work is innovative, as it aims to identify a novel pathway by which the host response contributes to disease severity during CDI. Successful completion of these studies will establish a complete mechanism by which IL-23 signaling acts in a pathogenic manner during CDI, identifying potential host-based targets for drug intervention.